The invention relates to communications systems, more particularly to adapting receiver signal processing parameters based on the current service and frequency band used.
The cellular telephone industry has made phenomenal strides in commercial operations throughout the world. One important step in the advancement of radio communication systems has been the change from analog to digital transmission. Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and WCDMA telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS standard. This application focuses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
Equally significant is the choice of an effective digital transmission scheme for implementing next generation technology. Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, and the like, are being provided by, for example, cellular carriers using the digital cellular system infrastructure. An important feature desired in these new systems is increased traffic capacity, and efficient use of this capacity. It is also important for the portable devices in such systems to provide high quality service while conserving energy to whatever extent possible, since they are very often powered by batteries.
Recent efforts at providing such systems have focused on the use of WCDMA techniques. In a WCDMA system, multiple users utilize the same radio spectrum simultaneously. From the point of view of a receiver in a WCDMA system, a received signal comprises a desired signal (i.e., a signal intended to be received by that particular receiver) and a high level of noise. To enable the receiver to extract the desired signal from the received signal, information intended for that receiver is “spread” by combining (e.g., by multiplying) the information with a much higher bit rate known signature sequence. The signature sequence is unique to this particular receiver. One way to generate the signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized user.
Because each active transmitter is utilizing the same process, a plurality of spread information signals modulate a radio frequency carrier, for example by binary phase shift keying (BPSK), and as said before, are jointly received as a composite signal at the receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique signature sequences, and the corresponding information signal can be isolated and despread. If quadrature phase shift keying (QPSK) modulation is used, then the signature sequence may consist of complex numbers (having real and imaginary parts), where the real and imaginary parts are used to modulate respective ones of two carriers at the same frequency, but ninety degrees out of phase with respect to one another.
Traditionally, a signature sequence is used to represent one bit of information. Receiving the transmitted sequence or its complement indicates whether the information bit is a +1 or −1, sometimes denoted “0” or “1”. The signature sequence usually comprises some number, N, bits, and each bit of the signature sequence is called a “chip”. The entire N-chip sequence, or its complement, is referred to as a transmitted symbol. The conventional receiver, such as a RAKE receiver, correlates the received signal with the complex conjugate of the known signature sequence to produce a correlation value. Only the real part of the correlation value is computed. When a large positive correlation results, a “0” is detected; when a large negative correlation results, a “1” is detected.
A number of efforts to standardize the use of WCDMA techniques in mobile communication systems exist. One such effort is being accomplished by the 3GPP. The term “third generation” refers to the fact that so-called second-generation radio access technology brought mobile telephony to a broad market. By contrast, third-generation radio access technology extends beyond basic telephony: a common, Internet Protocol (IP)-based service platform will offer mobile users an abundance of real-time and non-real time (traditional data) services.
Typical services with real-time requirements are voice and video, as well as delay-sensitive applications, such as traffic-signaling systems, remote sensing, and systems that provide interactive access to World Wide Web (WWW) servers. As explained in, for example, F. Müller et al., “Further evolution of the GSM/EDGE radio access network”, Ericsson Review vol. 78, no. 3, pp. 116-123 (2001), the challenge is to implement end-to-end services based on the Internet Protocol (IP). The main benefit of running IP end-to-end—including over the air interface—is service flexibility. Indeed, flexibility more or less eliminates dependencies between applications and underlying networks, for example, access networks. To date, cellular access networks have been optimized in terms of voice quality and spectrum efficiency for circuit-switched voice applications. However, for services such as IP multimedia, which includes voice, the main challenge is to retain comparable quality and spectrum efficiency without decreasing service flexibility. Today, for example, we can suffer considerable protocol overhead when we bridge the air interface with real-time protocol (RTP), user datagram protocol (UDP) or IP packets (which carry media frames). Needless to say, this runs counter to the goal of spectrum efficiency. To achieve spectrum efficiency, we can instead characterize different packet data streams in terms of bandwidth and delay requirements. Characterization of this kind is useful when implementing admission access algorithms that accommodate multiple user data streams in available spectrum. Different methods of limiting data (such as header compression and session signaling compression) must also be applied to obtain adequate spectrum efficiency.
T. Hedberg and S. Parkvall, “Evolving WCDMA”, Ericsson Review vol. 77, no. 2 pp. 124-131 (2001) describes how, for the purpose of improving support for best-effort packet data, the 3GPP is working on an evolution of WCDMA known as high speed downlink packet data access (HSDPA). This enhancement to prior systems increases capacity, reduces round-trip delay, and increases peak data rates up to 8-10 Mbit/s. To achieve these goals, a new shared downlink shared channel (HS-DSCH) has been introduced. In addition, three fundamental technologies, which are tightly coupled and rely on the rapid adaptation of the transmission parameters to the instantaneous radio conditions, have been introduced with this channel: fast-link adaptation technology enables the use of spectral-efficient higher-order modulation when channel conditions permit (for example, during a fading peak), and reverts to robust QPSK modulation during less favorable channel conditions (for example, when experiencing a fading dip); fast hybrid automatic-repeat-request (ARQ) technology rapidly requests the retransmission of missing data entities and combines the soft information from the original transmission and any subsequent retransmissions before any attempts are made to decode a message; and fast scheduling of users sharing the HS-DSCH—this technique, which exploits multi-user diversity, strives to transmit to users with favorable radio conditions.
The general arrangement of a conventional receiver is illustrated by FIG. 1. Such a receiver may be used in user equipment (UE), such as a mobile terminal, personal digital assistant (PDA), and the like, communicating in a communication system. An antenna 110 provides a signal to a receiver front end 120, which down-converts the signal to lower frequencies that are more conveniently handled by the receiver. The down-converted signal is appropriately shaped by a filter 130 and variable amplifier 140, which typically has an adjustable gain. The gain of the variable amplifier 140 is automatically controlled by an automatic gain controller (AGC) 170 based on feedback of a portion of the amplified down-converted signal. The amplified down-converted signal is then typically provided to an analog to digital converter (ADC) 150 and additional processing components 160. The AGC 170 adjusts the variable amplifier's 140 gain to maintain the signal at a level that is within a usable range, i.e., dynamic range, of the receiver for further processing. The filter 130 structure and parameters are set to allow the extraction of the signal under the worst case scenario in terms of interference from other signals. For example, a significant limitation in high data rate communication systems is inter-symbol interference (ISI).
The dynamic range in different parts of the receiver is important due to requirements related to power consumption of the receiver. Since UE's are typically portable terminals that are battery powered, it is desirable to minimize power consumption of the receiver. It is well known in the art that by limiting the dynamic range of a received signal within the receiver, power conservation may be realized. For example, the dynamic range of a radio signal input to an ADC should be as limited as possible to enable the use of low resolution, low power ADCs in the receiver. Furthermore, power consumption in analog amplifiers and filter sections is directly proportional to the dynamic range of the input signal.
In conventional receiver architectures used in cellular communication systems (e.g., WCDMA), the digital filtering structure as well as the receiver gain parameters are fixed regardless of the type of service and/or frequency band used. That is, the various parameters that define the filtering and receiver gain applied to the received signal are based on worst case scenarios over all services and frequency bands for a given cellular communication system. Here, the term “services” includes speech service, HSDPA service, video services, and the like. Accordingly, for some combinations of services and frequency bands, the receiver current consumption is unnecessarily high given the received signal. Similarly, the receiver performance tends to be unnecessarily low in certain situations due to this worst-case design.
To illustrate this point, consider that the WCDMA standard will soon utilize frequency bands used by other cellular systems. For example, WCDMA will be deployed in the United States using a frequency in the 1900 MHz band (WCDMA 1900). On this band, however, GSM, and perhaps other cellular systems, may also be operating. The use of two or more systems operating within the same frequency band will present additional interference scenarios, above those expected in a standard WCDMA-only frequency band scenario. The presence of the additional interference will place more stringent requirements on the receiver chain, in terms of filtering and AGC parameters. The adjacent channel requirement for WCDMA 1900 requires sharp filtering, which introduces interchip interference (ICI). Such added interference impacts the performance of HSDPA service. In order to achieve the highest data rates provided in HSDPA, the receiver parameters will have to be optimized for such service. That is, the filtering requirements, sampling accuracy, gain parameters, and the like, should be optimized to prevent the WCDMA 1900 adjacent channel requirements from severely degrading HSDPA peak performance.
To employ a fixed receiver design that is optimized only for the worst case scenario over all frequency bands and services would produce unnecessarily high current consumption and/or reduce HSDPA performance in scenarios where better peak rates could be achieved. What is therefore needed is a method and receiver for adapting receiver signal processing parameters based on the current service and frequency band used to optimize the receiver performance and minimize current consumption.